Current Drug Targets - Volume 14, Issue 4, 2013

Androgens and the androgen receptor have been the focus of prostate cancer research since the early 1940s, when Huggins and Hodges demonstrated that removal of androgens caused advanced prostate cancer to regress. Since that time, a large number of androgen deprivation therapies have been developed in an effort to cure this disease, but prostate cancer remains one of the leading causes of cancer death in males worldwide. This is due in part to the emergence of castration- recurrent prostate cancer in patients with advanced disease who have failed androgen deprivation therapy. The androgen receptor is still a major player in castration-recurrent disease, and though much has been discovered since the early work of Huggins and Hodges regarding how prostate cancer cells manage to avoid the effects of androgen deprivation, survival times for men with advanced prostate cancer have changed only modestly. Research is now directed toward delineating the mechanisms of action of the androgen receptor under castrate conditions, whether through amplification of the AR, mutation, expression of splice variants, use of alternate signaling pathways, aberrant expression and activation of coregulators, or intratumoral androgen biosynthesis. Genome-wide association studies are also adding to the wealth of knowledge surrounding the androgen receptor, and with this knowledge comes the ability to design new drug therapies directed toward eradication of this disease.

The growth and dependence of Prostate Cancer (PCa) on androgen stimulation led to the use of castration to reduce circulating levels of androgens and anti-androgens to directly target the androgen receptor (AR) ligand-binding domain (LBD). However, castration-resistant prostate cancer (CRPC) resistant to anti-androgens invariably develops and can be associated with AR genomic aberrations (mutations, amplification) and/or an increase in AR mRNA expression. Efforts to more effectively target the AR in CRPC led to the rational design of CYP17A1 inhibitors and more potent antiandrogens. The front-runner 2nd generation rationally-designed therapeutics targeting the AR, abiraterone and enzalutamide have been shown to improve survival and clinical outcome for CRPC patients. Several other CYP17A1 inhibitors and anti-androgens are in clinical and preclinical development. However, patients ultimately progress and current evidence suggests that this can occur through reactivation of AR signaling. Several ongoing programs aim to develop LBDindependent therapeutic strategies that for example target the N terminus domain (NTD) of the AR or chaperone proteins. Rationally-designed approaches combining different strategies for targeting the AR or associated pathways also warrant clinical evaluation.

Prostate cancer (CaP) is the most frequently diagnosed cancer and leading cause of cancer death in American men. Almost all men present with advanced CaP and some men who fail potentially curative therapy are treated with androgen deprivation therapy (ADT). ADT is not curative and CaP recurs as the lethal phenotype. The goal of this review is to apply our current understanding of CaP and castration-recurrent CaP (CR-CaP) to earlier studies that characterized ADT and the molecular mechanisms that facilitate the transition from androgen-stimulated CaP to CR-CaP. Reexamination of earlier studies also may provide a better understanding of how more newly recognized mechanisms, such as intracrine metabolism, may be involved with the early events that allow CaP survival after initiation of ADT and subsequent development of CR-CaP.

The androgen receptor (AR) is a master regulator transcription factor in normal and cancerous prostate cells. Canonical AR activation requires binding of androgen ligand to the AR ligand binding domain, translocation to the nucleus, and transcriptional activation of AR target genes. This regulatory axis is targeted for systemic therapy of advanced prostate cancer. However, a new paradigm for AR activation in castration-resistant prostate cancer (CRPC) has emerged wherein alternative splicing of AR mRNA promotes synthesis of constitutively active AR variants that lack the AR ligand binding domain (LBD). Recent work has indicated that structural alteration of the AR gene locus represents a key mechanism by which alterations in AR mRNA splicing arise. In this review, we examine the role of truncated AR variants (ARVs) and their corresponding genomic origins in models of prostate cancer progression, as well as the challenges they pose to the current standard of prostate cancer therapies targeting the AR ligand binding domain. Since ARVs lack the COOH-terminal LBD, the genesis of these AR gene rearrangements and their resulting ARVs provides strong rationale for the pursuit of new avenues of therapeutic intervention targeted at the AR NH2-terminal domain. We further suggest that genomic events leading to ARV expression could act as novel biomarkers of disease progression that may guide the optimal use of current and next-generation AR-targeted therapy.

DNA damage and genetic rearrangements are hallmarks of cancer. However, gene fusions as driver mutations in cancer have classically been a distinction in leukemia and other rare instances until recently with the discovery of gene fusion events occurring in 50 to 75% of prostate cancer patients. The discovery of the TMPRSS2-ERG fusion sparked an onslaught of discovery and innovation resulting in a delineation of prostate cancer via a molecular signature of gene fusion events. The increased commonality of high-throughput sequencing data coupled with improved bioinformatics approaches not only elucidated the molecular underpinnings of prostate cancer progression, but the mechanisms of gene fusion biogenesis. Interestingly, the androgen receptor (AR), already known to play a significant role in prostate cancer tumorigenesis, has recently been implicated in the processes resulting in gene fusions by inducing the spatial proximity of genes involved in rearrangements, promoting the formation of double-strand DNA breaks (DSB), and facilitating the recruitment of proteins for non-homologous end-joining (NHEJ). Our increased understanding of the mechanisms inducing genomic instability may lead to improved diagnostic and therapeutic strategies. To date, the majority of prostate cancer patients can be molecularly stratified based on their gene fusion status thereby increasing the potential for tailoring more specific and effective therapies.

Androgen receptor activity is essential for prostate cancer development and progression. While there are classically defined roles for the retinoblastoma (Rb) and p53 tumor suppressor pathways in maintenance of cell cycle control and the DNA damage response, recent studies have demonstrated a direct role of these two pathways in regulating AR expression and function. While the role of Pten deregulation in prostate cancer has provided much insight in to the mechanisms underlying prostate cancer initiation and progression, emerging roles for Rb and p53 are likely to further expand upon our understanding of tumor suppressor/nuclear receptor interaction. As disconnecting mitogenic signaling from ARmediated gene transcription underlies the progression to castrate resistant prostate cancer (CRPC), functional inactivation of these two tumor suppressor pathways represents one mechanism through which AR protein levels can be upregulated and AR-mediated gene transcription can become aberrant. Importantly, recent advances in small molecule inhibitor design and discovery have led to the identification of agents capable of targeting these two prominent pathways and restoring the function of deregulated wild-type Rb and p53 protein. While such agents have undergone extensive study in many solid tumor types, the additional importance of Rb and p53 in restraining transcription of the AR gene within the prostate provides impetus for examining how loss of these two tumor suppressor proteins can facilitate transition of prostate cancers to CRPC. As will be reviewed in this article, restoration of Rb and p53 functions are not only important in regard to shortterm cell cycle regulation and response to genomic stresses, but likely have direct implications for deregulation of the AR locus.

The androgen receptor (AR) is a steroid hormone receptor that functions as a ligand-dependent transcriptional factor, which plays a key role in the development and progression of prostate cancer. Recent advancement in highthroughput technologies including microarrays and deep-sequencing provides unbiased genome-wide knowledge on the AR signaling including datasets for androgen-regulated gene expression and genomic binding sites for AR. In the present review, we will briefly summarize the main features of the AR signaling as well as the individual AR target genes identified by the integration of multiple datasets in prostate cancer. Cap analysis gene expression (CAGE) is also featured as a unique transcriptome method, which particularly determines the androgen-dependent transcription start points in prostate cancer.

Androgen receptor (AR) action is critical for prostate cancer (CaP) progression, but is not inhibited fully by available androgen deprivation therapy (ADT). One of the limitations to current ADT is that it targets all androgen action in CaP, and other, cells irrespective of clinical relevance. The resulting off-target effects are responsible for ADTassociated side effects that affect negatively a patient's quality of life. Isolation of the AR-dependent events that drive CaP progression may lead to novel forms of ADT that are at least as effective but more selective. Here, an approach is described that starts from insights in the basic mechanism(s) by which AR regulates target gene expression to identify novel drugable targets downstream of AR. Exploration of the molecular events that underlie androgen regulation of the ARassociated coregulator FHL2 led to the isolation of a novel indirect mechanism of androgen action that is mediated by the secondary transcription factor Serum Response Factor (SRF). Using a combination of oligoarray and in silico analyses, an SRF-dependent fraction of AR action was identified that is enriched in CaP tissues, is able to discriminate between benign and malignant prostate, and correlates with aggressive disease and biochemical failure. The RhoA signaling axis, a wellknown upstream stimulator of SRF action that harbors drugable targets, conveyed androgen-responsiveness to SRF, and was activated in CaP where it correlates with increased CaP aggressiveness and poor outcome after surgery.

Diabetic nephropathy (DN) continues being the primary cause of chronic hemodialysis and terminal renal disease worldwide. At tissue levels the DN occurs with glomerulopathy affecting the integrity of the filtration barrier and with an extensive glomerular and tubule-interstitial fibrosis. Current available therapeutic approaches have only demonstrated a modest effect on progression of kidney injury. Therefore, more research concerning the pathomechanisms and possible interventions are needed. Interestingly, in the last years it has been documented that DN progresses with growing levels of the nucleoside adenosine. This finding increased the interest in the events controlling the extracellular levels of the nucleoside. While the metabolism of extracellular ATP and cyclic AMP are well recognized sources, evidences regarding the role of the equilibrative nucleoside transporters in controlling adenosine availability and promoting diabetic glomerulopathy have recently acquired a pivotal role. The physiological effects of nucleoside are mediated by the P1 family of adenosine receptors. It has been shown in vivo that the use of an antagonist of the A2B receptor subtype can block the most remarkable early alterations seen in diabetic glomerulopathy. Furthermore, using models of chronic kidney injury it was demonstrated that fibrosis can also be blocked using treatment with the antagonist of A2B receptor subtype. This review highlights these findings that correlate the activity of a low affinity adenosine receptor with an increase in the ligand availability in the pathological state. In addition, we discuss the possible therapeutic interventions of adenosine signaling with regards to DN treatment.

Ischemic stroke is the leading cause of mortality and morbidity worldwide for which the assemblage of therapeutic interventions has remained outstandingly limited. Several new insights into the causes of neuronal death during ischemic event have led to the identification of some important novel targets for intervention. This article highlights some of the promising protein targets, which are in the validation process and have the potential to get translated into viable neurotherapeutics against ischemic stroke.

A wealth of preclinical and clinical literature has established functional associations of CNS dopamine (DA) and its multiple G protein-coupled receptor (GPCR) types in the integration of key neurological processes linked to complex behavioral activities. Conversely, an equivalent vast literature supports the role of aberrant CNS DA expression and DA receptor signaling in the etiology and persistence of major psychiatric illnesses and has established selective targeting of DA-ergic systems as a cornerstone of pharmacotherapeutic intervention and current neuroleptic drug development. The present short review focuses on potential functional/behavioral alterations linked to polymorphisms in the primary DNA sequence of the DA receptor type 4 (DRD4) gene in reference to major psychiatric illnesses. The potential clinical relevance of major polymorphisms of the DRD4 gene are discussed within the context of practical aspects of typical and atypical neuroleptic drug usage within afflicted populations of psychiatric patients. It is anticipated that additional complementary molecular, biochemical, and behavioral studies of DRD4 gene polymorphisms will provide essential information for selective targeting of heterogeneous populations of CNS D4 receptors and advance drug discovery and therapeutic development efforts for highly efficacious treatment of psychiatric illnesses.